The present invention relates to a carrier chrominance signal processing circuit for a video signal playback apparatus such as a video tape recorder, and in particular to a carrier chrominance signal processing circuit whereby the level of noise in a playback carrier chrominance signal is reduced by employing a feedback comb filter.
The use of a comb filter for reducing noise components in a playback carrier chrominance signal of a video tape recorder, particularly noise resulting from crosstalk generated by the record/playback process, is well known. Such a comb filter is based upon a delay circuit which produces a delay equal to an integral number of horizontal scanning intervals. In its simplest form such a comb filter consists of a single delay circuit which is coupled to receive the playback carrier chrominance signal, and means for combining the output signal from the delay circuit with the input carrier chrominance signal to thereby provide a filtered output signal. However improved filter performance can be achieved by implementing feedback of part of the latter output signal to the input of the filter, such an arrangement being referred to in the following as a feedback comb filter.
In the following, the designation "vertical coherence" will be applied to a condition of a carrier chrominance signal or a luminance signal in which the data contents of two successive horizontal scanning intervals of the carrier chrominance signal or the luminance signal would produce a substantially vertically aligned pattern on a CRT, when displayed. For example vertical coherence would be exhibited by all of successive pairs of scanning lines of a carrier chrominance signal if the data therein will result only in one or more vertical color lines being displayed. Similarly, vertical coherence would be exhibited by successive pairs of luminance signal horizontal scanning intervals if the data therein will result only in one or more vertical dark or light lines being displayed. Similarly, the designation "vertical non-coherence" will be applied to a condition of a carrier chrominance signal or a luminance signal in which the data contents of two successive horizontal scanning intervals of the carrier chrominance signal or the luminance signal would produce a vertically non-aligned pattern on a CRT after having been processed to be displayed. For example vertical non-coherence would be exhibited by a pair of successive horizontal scanning lines in a carrier chrominance signal if the data therein will result in one or more horizontal color lines or line segments being displayed by only one of these scanning lines, or in the two scanning lines respectively producing displayed line segments which are not mutually vertically aligned.
In practice, the descriptions "vertical coherence" and "vertical non-coherence" are not used in an absolute sense, but are of variable degree and can be judged with reference to a specific threshold level, as described hereinafter.
It has been found that the requirements for optimizing the degree of feedback within a feedback comb filter used for a playback carrier chrominance signal in a video playback apparatus (i.e. such as to minimize the amount of blurring in the vertical direction of the resultant displayed image) will vary, depending upon the current conditions of vertical coherence and vertical non-coherence exhibited by the playback carrier chrominance signal and luminance signal. More specifically, although the use of a feedback comb filter provides a greater improvement of the signal/noise ratio of a playback carrier chrominance signal than does a non-feedback comb filter, the feedback comb filter has the disadvantage of producing vertical blurring of the displayed image under certain conditions. In general, if both the luminance signal and carrier chrominance signal are substantially vertically non-coherent, as defined hereinabove, then the use of a feedback comb filter to process the playback carrier chrominance signal will result in vertical blurring of the displayed image, with the amount of blurring being decreased if the feedback factor of the comb filter is reduced. Conversely, if both the luminance signal and carrier chrominance signal are substantially vertically coherent then such vertical blurring will not be produced by the use of a feedback comb filter. For this reason, it has been proposed to continuously monitor the degree of vertical coherence of the luminance signal, and to control the degree of feedback in the feedback factor in accordance with that vertical coherence. FIG. 1 is a general block diagram of a system known in the prior art whereby the feedback factor of a feedback comb filter is varied in accordance with the amount of vertical coherence of the playback luminance signal, the system being utilized in a video tape recorder. In FIG. 1, video data from a magnetic tape 1 is produced as a video signal from a playback head 2. This signal is transferred through a preamplifier 3 to a high-pass filter (HPF) 4, which separates the luminance signal from the video signal. The luminance signal is transferred through a playback luminance signal processing circuit 5 to a Y vertical coherence signal detection circuit 6 (where "Y" has the generally accepted significance in color television technology of "luminance signal", and "C" as used hereinafter has the generally accepted significance of "chrominance signal"), which produces a detection signal indicating vertical non-coherence.
The output signal from preamplifier 2 is also supplied to a low-pass filter LPF 7, which separates the chrominance carrier signal. The chrominance carrier signal is supplied to a chrominance signal processing circuit 8, which performs processing (i.e. up-conversion) to produce a 3.58 MHz carrier chrominance signal (in the NTSC system). The processed carrier chrominance signal from chrominance signal processing circuit 8 is then supplied to a noise reduction circuit 9, which is based on a feedback comb filter. In the the feedback comb filter noise reduction circuit 9, the degree of feedback is controlled by a control signal in accordance with an output signal from the Y vertical coherence signal detection circuit 6, which detects the degree of vertical coherence (as defined hereinabove) of the luminance signal in successive pairs of horizontal scanning lines. Variation of the amount of feedback in the feedback comb filter of noise reduction circuit 9 is performed such as to lower the levels of noise components in the carrier chrominance signal. Following this noise reduction processing, the carrier chrominance signal is combined with the luminance signal from the playback luminance signal processing circuit 5, in a mixing circuit 10, and the resultant composite video signal is applied to an output terminal 11.
The assignee of the present invention has previously proposed a carrier chrominance signal processing circuit, basically applicable to the system shown in FIG. 1, in which the degree of feedback of the feedback comb filter is controlled in accordance with the status of vertical coherence of the playback luminance signal. This circuit is described in Japanese patent application No. 59-247538, which was published on June 14, 1986 with publication No. 61-126890 and is shown in FIG. 2, and corresponds to the Y vertical coherence signal detection circuit 6 and the noise reduction circuit 9 shown in FIG. 1. In FIG. 2 the 3.58 MHz carrier chrominance signal designated as PB C is applied to input terminal 12, for example from the chrominance signal processing circuit 8 in FIG. 1, and has the noise components thereof reduced by a feedback comb filter. This is made up of an adder/subtractor circuit 13, a 1 H delay circuit (1H DLY) 14 (where H denotes the duration of one horizontal scanning interval), an adder circuit 15, a limiter 17, and a feedback factor control circuit 16 which can comprise for example a voltage-controlled amplifier. The output signal from this feedback comb filter circuit is supplied to an output terminal 18. The luminance signal designated as PB Y which is applied to terminal 19, e.g. from luminance signal processing circuit 5 in FIG. 1, is supplied to a Y vertical coherence detection circuit 23, which is made up of an adder/subtractor 21, a detector 22 and a 1H delay line 20. The Y vertical coherence detection circuit produces a detection signal indicating the presence or absence of vertical coherence, and this signal is supplied to the feedback factor control circuit 16 to control the feedback factor of the feedback comb filter, which is designated in the following as K. Specifically, when the luminance signal is detected as having vertical coherence, then the feedback factor K of the feedback comb filter is controlled, by the detection signal from detection circuit 23 acting on the feedback factor control circuit 16, to be a maximum. If the luminance signal is detected as being non-coherent, then the feedback factor K is controlled to be lower than the value of K which is produced when vertical coherence of the luminance signal is detected. This lower value may be zero, in which case the feedback factor will effectively be caused to function as a non-feedback comb filter. The limiting level set by the limiting factor S of limiter 17 ( where S&lt;&lt;1) is established such that the output signal from limiter 17 is held to within 10% of the maximum input signal level thereto. This output signal is then multiplied by the feedback factor K that is determined by the feedback factor control circuit 16, so that the effective feedback factor of the feedback comb filter becomes K.S. The amplitude of the feedback signal is therefore substantially smaller than in the case of prior art circuits of this type. The feedback signal is then subtracted from the carrier chrominance signal in adder/subtractor 13, and the resultant output signal is applied to terminal 18.
As a result of the feedback factor of the feedback loop in this circuit being reduced from K to K.S, the cut-off frequency of the circuit is increased. Thus, the filter circuit has an improved capability for following rapidly occurring signal variations.
Although the adder/subtractor 13 is shown to operate as a subtractor in FIG. 2, it actually functions as an adder, because there is a phase difference of 180.degree. between its input signals.
With the circuit of FIG. 2, a reduction of color blurring in the vertical direction, when the luminance signal and the carrier chrominance signal are not vertically coherent, is attained due to the reduction in the feedback factor of the feedback comb filter. However under some circumstances, in particular when the input signal contains noise components which exceed the limiter level of limiter 17, the circuit cannot effectively function to limit color blurring in the vertical direction or to increase the S/N ratio.